The present invention relates to a self-penetrating fastening device consisting of a rivet adapted to be impacted by a punch and a counterpressure pad (die), and more particularly to a rivet consisting of a head and shank with a central cavity. The free end surface of the rivet (i.e. remote from the head) does not completely penetrate a workpiece receiving it.
A self-penetrating fastening system is described in German Patent DE 39 42 482 C1. In the embodiment described in this patent, two plates of different thicknesses, i.e., parts of the wall of a container, are connected to each other, with particular concern being given to water and gas tightness. Corrosion stability is also taken into account in this state of the art.
If such self-penetrating fastening systems are also to be used in the area of automotive technology, for example in order to connect body sheets together, other essential factors are involved in addition to corrosion stability. Because of the high mechanical stresses occurring, the strength of the connection of two plates is of critical importance. In addition, depending on the location where the connection is applied (i.e., uncovered body areas), it is desirable to make the top of the rivet head as nearly flush as possible with the surrounding plate surface after connection. The process of producing the riveted connection should damage the surface areas in question as little as possible (e.g., through deformation resulting from warping of the sheet), so as to hold to a minimum any finishing operations required to improve the appearance.
The state-of-the-art riveted connection does not meet these additional requirements to the extent desired.
It therefore is the principal object of the present invention to develop the state-of-the-art self-penetrating fastening system in such a manner that the riveted connection produced by the system will meet strict requirements with respect to its strength and so that the surface of the workpiece will be minimally affected.
As a result of the design of the shank cavity in the area of the free end surface, i.e., remote from the head, the cavity being substantially conical in this area, as the rivet penetrates a workpiece more deeply the resulting higher expansion force causes the radial movement of the rivet shank outward (affects expansion) to reach an order of magnitude such that an expansion angle greater than 45xc2x0 may be reached. This process results in an undercut that increases the strength, and the direct tensile strength in particular, since the end area of the rivet shank projects radially further than the convexity of the lower sheet adjoining the outside surface of the shank. The formation of the rivet shank in the area of the free end surface and the essentially rounded transition of the shank into the rivet head results in minimal deformation of the upper sheet during penetration. Further, if the rivet head has a flat top, the rounded transition enables this top to be embedded so as to be flush with the surrounding surface of the sheet and so that the circular notch marking the transition of rivet head to sheet surface is very narrow and extremely shallow.
In addition, the circumferential wall of the die cavity is designed so as to be radially movable. Especially in the case of thin sheets, this provides a supporting effect during cutting of the top sheet without inhibiting the spreading of the rivet shaft in the bottom plate since the spring-mounted groove blocks can move radially outward as required.